Knock-knock jokes hold a special fascination for preschoolers, probably because it’s their first try at the kind of humor that will evolve, inevitably, into middle-school fart jokes. It’s not as easy as it looks, though. Practice makes perfect in knock-knock land.

The basics: Tim’s come to see me with a mild case of poison oak. His mom encourages Tim to tell me his new joke. I’m game. I like knock-knock jokes, especially as told by four year-olds.